As much as I like the seats, in the current cold climate here I missed the comfort of my heated stock seats.
So I started looking and found on eBay an aftermarket kit for heated seats.
It's basically four thin fabric sheets with the heating elements inside, two switches and lots of wire.

So now how to install them?
I got lucky as the leather seat cushions of my GT3 seats all have a velcro flap on the side where they open and then you find some dense foam inside. So basically it was very easy:
Open flap, take foam out, put the heating elements on top of the foam pad, stuff it carefully back inside, close flap.

For the electric wiring:
Initially I wanted to use my OEM switches for the seat heaters for a complete OEM look, but soon figured out that this is not straight forward at all.
So I went with the supplied switches and decided to install them in the batwing.
I found a used batwing (I wanted to keep my original intact), drilled the holes for the switches, got the power from the phone connector in the center console. The switches in the batwing almost look stock.